Electric arc furnace (hereafter referred as EAF) dust contains elements such as zinc, iron, lead, aluminum, chromium, cadmium, manganese, sodium, potassium, magnesium and calcium. The zinc in the dust is present as zinc oxide and zinc ferrite. EAF dust is regarded as a hazardous waste because of the presence of significant amounts of leachable compounds of zinc, lead, cadmium and chromium. This classification as a hazardous waste means that the EAF dust can not be disposed off with out treatment. EAF dust that is obtained from processing ferrous steel scrap will have a K061 “Hazardous Rating” as per US Environmental Protection Agency Standards. Salvaged ships, structural steel, galvanized steel and primary automobile scrap are the major sources of feed to the smelting furnaces. The problem with untreated EAF dust in US and various other countries is substantial. Many tons of EAF dust are presently stored in US at the existing furnace operations at a rate of about 500,000 to 750,000 tons per year. The pyrometallurgical processors can only process about 250,000 to 300,000 tons per year, and this ads to the stockpile at the rate of 250,000 to 500,000 tons per year.
The various options that are available to the generator includes disposal, recycle and/or recovery of valuable metals. A current major route of disposal is stabilization and landfilling. However, stabilization and landfilling loses the potential of recovering zinc and other metals. The other two options are: a) Reducing the dust with coal, methane or hydrogen at an elevated temperature and separating the condensable zinc vapor from a non-volatile slag using pyrometallurgical techniques, b) Removing the zinc by a hydrometallurgical process.
The recycling of electric arc furnace dusts by pyrometallurgical and hydrometallurgical techniques have been the subject of many studies. Pyrometallurgical processes require reducing agents and high temperatures and generally produce a crude zinc oxide of low commercial value. On the other hand hydrometallurgical processes can produce high quality metallic zinc or zinc oxide, but most of them can not leach zinc completely from the zinc ferrite phase.
The hydrometallurgical methods for the treatment of EAF dusts could be classified into acidic and alkaline leaching systems. The acid leaching systems that are studied by several inventors are the sulfuric acid and hydrochloric acid systems. These two acids were investigated on a stand alone basis and also in the presence of additives. Most processes using sulfuric acid require high temperature and high-pressure conditions in order to avoid jarosite formation. In most of the hydrometallurgical inventions atmospheric leaching of zinc ferrite material is difficult and therefore several investigations using expensive pressure leaching techniques were patented.
In a Canadian patent 2076025 recovery of metal values from zinc plant residues containing zinc ferrites is disclosed. The process consists of partial dissolution of zinc, copper, iron and some impurities with spent H2SO4 at 70-100° C. Then mix the leach slurry with zinc concentrate and subject to oxidative leach followed by reductive leach in a hot sulfuric acid system. The resulting residue is then subjected to flotation to recover the undissolved zinc concentrate. The flotation tailings were subjected to pressure leaching using gaseous sulfur dioxide. The process described consists of several leaching, flotation and pressure leaching steps and therefore is capital intensive. In Canadian patent 2240450 leaching of zinc ferrite with peroxysulphuric acid was disclosed. In Canadian patents 1217638 and 2104736 two stage leaching with sulphuric acid was disclosed. In the second stage sodium, potassium or ammonium ions were added for jarosite formation. The amount of sulphuric acid added in the second stage was slightly in excess to the amount of ferrite material present. In U.S. Pat. No. 5,286,465 use of hot concentrated sulfuric acid leaching was disclosed.
In Canadian patent 1212841 a process for the extraction of zinc from zinc ferrite residues by pressure leaching with sulphuric acid in an autoclave system was disclosed. In Canadian patent 1176853 zinc ferrite was combined with zinc sulphide ore and then pressure treated at elevated temperature to dissolve zinc from the ferrite as well as the sulphide ore. In another Canadian patent 1112880 leaching with aqueous sulfuric acid at an elevated temperature of 140 to 250° C. and 3-40 atm pressure was carried out. These high temperature and high pressure processes are in general are more expensive to the atmospheric leaching processes. In U.S. Pat. No. 4,610,721 atmospherically leaching the steel plant dust in a first stage wherein an amount of steel plant dust is mixed with an amount of acidic zinc sulfate solution to leach zinc therefrom. Solution pH was controlled between 2.5 and 3.5. Following thickening additional sulfuric acid or the spent electrolyte was added and subjected to pressure leaching.
Processes using hydrochloric acid solutions and mixed acid solutions containing HCl are discussed. In Canadian patent 2156295 and in U.S. Pat. No. 5,336,297 a process for the treatment of Electric Arc Furnace dust was disclosed. This process consists of atmospheric ferric chloride leach followed by treatment in autoclave at elevated temperature and pressure for the conversion of goethite to easily filterable hematite. In U.S. Pat. No. 5,709,730 leach solution was made of calcium chloride and HCl and leaching was conducted under controlled pH of 2.6 at an elevated temperature and pressure in an oxygen environment. Mixed oxides were recovered through precipitation by lime addition.
In U.S. Pat. Nos. 4,614,543 and 4,610,722 use of mixed lixiviant system made of HCl and H2SO4 was disclosed. The leaching of zinc was reported to be 70 to 71%. In U.S. Pat. No. 4,572,771 steel plant dust was leached with hydrochloric acid to provide pregnant liquor containing zinc, iron and lead. Zinc was electrowon and HCl was regenerated. All of the above processes use solvent extraction technology for zinc extraction followed by electrolysis of zinc strip liquor to produce zinc metal. In U.S. Pat. No. 6,338,748 a process was disclosed where hot acid leach containing 37-74 g/L of HCl and 104-270 g/L of ZnCl2 is used. This process claims the dissolution of both zinc oxide and zinc ferrite phases.